The number one biggest mistake in writing for the web is not understanding the people who will be reading the content. I see this happen all the time in the content teams that I help, and it can lead to a lot of problems. If you don't really know the person you're writing for, how can you craft the content to meet their needs?
Before you write anything, answer these three questions. Who are you writing this for? What do you they want?
And what do you want? First, who is this for? Which user groups or personas?
Determine how much they know about the topic. Once you've figured this out, you'll be better able to identify the likely gaps in your users' knowledge and fill those gaps in. Answering this question will also help you determine what technological terminology your readers will or won't be familiar with.
For example, imagine we're writing a description of a B-to-B product. If we know that a purchasing manager will be reading the topic and they may not have as much expertise in the topic, we might want to go out of our way to explain and define technical terms in more detail. The only way to identify and avoid using jargon is to know what your users will know.
Second, determine why somebody would be reading this. What do they want out of it? Are they looking to get a specific fact?
Are they trying to trying to understand the subject matter? Are they trying to decide whether or not to interact with us, or to make up their mind about the company? The answers to these questions will help you format and structure your content so that it can best serve that goal.
Once I was helping a government agency client answer these questions about their content. They had an article on their external site, and I asked them to consider why would someone seek out and read this? What would they be looking for, and what would they get from this?
They ended up realized that, actually, the only people who would get something from that particular piece of content would be employees of that agency. So they decided that that article had no business being on the external site and it should be moved to the intranet. Finally, what do you or your organization want from this piece of content and why are you writing it?
What do you hope will happen as a result? Essentially, what impact do you want this to have on the reader? Are you looking to drive more traffic to the site and attract attention?
Do you want the reader to decide to buy your product or sign up for your service? Do you want them to contact your sales team? Answering these questions will help you set your goals and the success criteria for the piece.
Actually accomplishing those goals involves combining the answers to this question along with the answers to the first two questions. How can you help your readers to get what they need, which also supports your organization's goals while delivering that information in exactly the right way for that specific person? Understanding who you're writing for and shaping the content to fit them is the core of UX writing.